Liquid-cooled multi-cylinder engines are generally cooled by forming a water jacket outside the cylinder walls forming the cylinder, and circulating a cooling liquid through a cooling passage formed between these walls.
In a particular cylinder block known as a Siamese cylinder block, the cylindrical engine cylinders are separated by a short interval, and the adjacent cylinder walls are joined together, so hollow areas are formed in the cooling passage at the sides of the joining members.
A horizontal section of the cooling liquid circulation passage therefore appears as a plurality of arcs formed by the bulge of the cylinders, these arcs being joined by the hollow areas. At the points where these hollow areas are situated, the cooling liquid passage is bent at a sharp angle. Because these bends obstruct the smooth flow of cooling liquid, an undesirable amount of heat is produced. FIGS. 16a and 16b show the distribution of the cooling liquid flowrate and the distribution of the rate of heat transmission under these conditions. It is seen that where the hollow areas 6 are located, both the flowrate of the cooling liquid, and the heat transmission rate are reduced.
Tokkai Hei 4-136461 published by the Japanese Patent Office in 1992 proposes decreasing the width of the water jacket midway along its length so as to increase the flowrate of cooling liquid through the hollow areas.
However, in this case, the cross-sectional area of the water jacket varies sharply, causing increased resistance to fluid flow and increased load on the water pump circulating the cooling liquid.
Moreover, as the cylinder block is generally of cast iron construction, the thickness of the insert used for forming the passage during the casting process becomes smaller at points where the cross-section of the passage undergoes a large variation. Thus, the strength of the insert tends to be insufficient, and renders casting difficult.